[BUGFIX] Tests: Replace objectManager with mock in ext:extbase

The usage of native objectManager in Extbase' BaseTestClass introduces
a dependency to the systems database. In order to reduce the unit test
bootstrap this dependency must be resolved.

Change-Id: I862664d4ca032d18f22b18677816bed1e1bc2a4a
Resolves: #56936
Related: #51436
Releases: 6.2
Reviewed-on: https://review.typo3.org/28421
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind